Fix default config resolver

This aligns the code of
`extension.GdbConfigurationProvider.resolveDebugConfiguration` with
`extension.GdbConfigurationProvider.provideDebugConfigurations`, so
that the debugger can be launched directly in workspaces that lack a
`launch.json`.

src/extension.ts

@@ -38,6 +38,9 @@ class GdbConfigurationProvider implements vscode.DebugConfigurationProvider {
         if (config.request === undefined) {
             config.request = "launch";
         }
+	if (config.preLaunchTask === undefined) {
+	    config.preLaunchTask = "SuperBOL: build (debug)";
+	}
         if (config.target === undefined) {
             config.target = "${file}";
         }